Dispersed Energy Storage

The U.S. electric grid has several challenges to overcome for continuous delivery of uninterrupted power. Critical needs include the upgrade or replacement of aging electrical infrastructure, increased intelligence and communication between components on the electric grid, and increased reliability of power for electric customers. In addition, the increasing desire for renewable energy places challenges on grid operators as these sources of energy are intermittent. Accurate prediction of their availability is difficult, and curtailment of their output may be needed to keep grid frequency stable.

Energy storage has been identified as a means to address some of these issues. However, determining an economically viable and technically sound solution has proven elusive. American Electric Power (AEP) has researched several energy storage technologies to determine which technologies can help overcome certain electric grid challenges.

The First Large-Scale NaS Battery

In 2006, AEP deployed the first utility-grid scale sodium-sulfur (NaS)-based energy storage system in the United States, near Charleston, West Virginia. A 1-MW, 7.2-MWh NaS battery was deployed. In its first three years of operation, the battery provided approximately 1 MW of load leveling during hot summer days, improved the feeder load factor by 5% and reduced the oil temperature of the associated substation transformer. This energy storage system was successful in deferring the need to build a new substation.

Load leveling provides one of the largest cost benefits for energy storage systems on the distribution portion of the electric grid because it allows capital deferral of upgrading or replacing substation electrical equipment. However, this benefit may not be enough to completely cover the cost of deploying an energy storage system.

To fully rationalize the cost of deploying energy storage systems, benefits have to be realized from several applications of the technology. In addition to load leveling for capital deferral, these benefits include providing backup power, energy arbitrage, system frequency regulation, the reduction of system losses and the integration of renewable sources of energy. Most of these benefits compete for the same energy and power out of the battery, so analyses have to be performed to prioritize the benefits, and some energy allocation may be necessary for the applications chosen to be deployed.

Building Upon NaS Success

After successfully deploying the Charleston energy storage system, AEP decided to deploy more installations and added the ability to provide backup power as another application to increase the benefit provided by the NaS-based technology. In cooperation with S&C Electric Co., AEP deployed three 2-MW, 14.4-MWh systems, commissioned in 2009. These projects were partially funded by the U.S. Department of Energy (DOE) through Sandia National Labs. The NaS-based energy storage systems provide 480 V and are connected to a step-up transformer to attain 12.47-kV or 34.5-kV circuit voltage.

For AEP, the new energy storage systems provided load-leveling benefit to the substation as well as backup power to customers during certain faults on the electrical system. The maximum area that could be served by the battery was determined and broken into zones, creating flexibility in providing backup power to customers.

Each zone is segmented by intelligent reclosers or switches. During an outage, the last load information is recorded, and the battery comes on-line to energize as many customers as possible. Based on that last load information, the number of zones that can be energized by the energy storage system is determined, and the switches open to disconnect the zones that cannot be energized by the energy storage system. The entire area can be energized by the battery, but during heavy load conditions, fewer zones may be picked up to ensure the maximum number of customers receives backup power.

As an example, on Nov. 1, 2010, a motor vehicle accident occurred that disrupted the three-phase power out of the Milton, West Virginia substation. The battery successfully came on-line and restored power to the entire island, approximately 700 customers, until the damage was fixed and power was restored from the station one hour and 17 minutes later.

In addition to distribution system batteries, AEP, specifically its transmission group, commissioned a 4-MW, 25-MWh NaS-based energy storage system in Presidio, Texas, in 2010. This is the largest such NaS-based deployment in the United States to date.

The Next Wave of Energy Storage at AEP

The NaS-based energy storage systems have been successful and provided lessons that allowed AEP to continue improving energy storage systems to meet customer needs and enable the utility to operate the distribution system more efficiently. There will always be a need for substation-based energy storage systems that are easily transportable to help overcome sustained outages. However, AEP is demonstrating another approach where energy storage systems can be more flexible to provide greater benefits to customers while still supporting grid functions.

This new approach is called Community Energy Storage (CES) — small-scale, battery-based energy storage systems that are connected to the secondary of utility distribution transformers to provide backup power to customers. Several systems can be aggregated to provide grid-level benefits to the utility.

Certain features of CES will enable the systems to provide benefits to both customers and grid operators:

Location. Energy storage systems that are located closer to customers increase the reliability of the systems to provide backup power. The closer those storage systems are located to customers, the less susceptible they are to weather conditions that may damage power lines when backup power is needed.

Buffer renewable energy. Storage systems located closer to customers can more readily buffer small-scale, customer-owned renewable sources of energy, like wind and solar.

Synergy with other industries. Today, utilities do not purchase a significant amount of battery-based energy storage systems. If utilities leverage technologies used by other industries, such as the automobile industry, there is greater opportunity to reduce cost by leveraging higher-purchased quantities than are presently available.

Smaller size. These units will be easier to install, operate and maintain. Outages to smaller-sized units are less critical to the operation of the electric grid.

CES Demonstration Project

As part of the AEP Ohio gridSMART demonstration project, partially funded by the DOE, the utility plans to deploy 80 CES units in Central Ohio. All the units deployed in the field will be connected on one circuit that provides power to approximately 1,700 customers. CES units will be associated with 20% of those customers. A CES control hub, located at the station, will be used to aggregate the units to provide circuit-level benefits such as load leveling and power factor correction.

The CES units will utilize lithium-ion batteries, similar to those that will be used by the automobile industry in electric vehicles. The decision was made to use lithium-ion because of its energy density characteristic and the potential for significant price reduction as a result of the volume that will be purchased by the automobile industry. AEP hopes to benefit from the leveraged price reduction so it can deploy more of these units in the future.

The batteries that will be deployed as part of the demonstration project are rated for one hour. However, to sufficiently meet grid needs and also provide backup power to customers, utilities will need batteries that can provide rated power for three hours to four hours.

CES Project Benefits

The CES project will demonstrate three main benefits:

Backup power to customers during outages on the electric grid

Load leveling at the substation

Power factor correction.

Backup Power

The CES units are rated 25 kW for power and 25 kWh for energy and will be located adjacent to padmount transformers near customers' homes. To provide backup power to customers during an outage, a CES unit will disconnect from the electric grid and provide power to the customers associated with that transformer. The unit will be capable of providing several hours of backup power, which is sufficient to handle most typical utility outages, because it will not be delivering fully rated power of 25 kW all the time.

Load Leveling

The CES units will be aggregated by a control hub at the substation to provide load-leveling benefit to the associated station transformer. The successful demonstration of this benefit will show that these systems can be used for capital deferral of equipment upgrade or replacement. A couple of different schemes will be used for load leveling:

Schedule-based load leveling. Using this method, utilities can simply perform load research analysis to determine a start time and choose a discharge schedule to perform load leveling.

Time-triggered load leveling. A more complex yet more intelligent method for load leveling involves using time of day as a trigger for load leveling. At a specific time of day, a determination is made on the necessity to perform load leveling. If the load is at or above a set level, load leveling will be performed on that day. This benefits the battery because a discharge will not occur on a low-peak day, thus extending the life of the battery by requiring fewer battery cycles and reserving more energy to provide backup power to customers. Load studies will need to be performed to determine both the time to analyze the need for load leveling and the actual time to start load leveling, which may differ.

Power Factor Correction

The last benefit that will be demonstrated during the CES project is the ability to provide capacitive support to the electric grid. The CES units will use four-quadrant inverters capable of discharging and charging real power and also providing inductive and capacitive power. The ability to provide capacitive support has great benefits to the electric grid. Most utility loads and power lines are inductive in nature, and significant levels of these inductive loads can cause voltage-drop concerns. Today, most utilities use capacitor banks to counterbalance these inductive loads. CES units will be able to provide the capacitive support to counter these inductive loads and enable the distribution system to run more efficiently.

Next Steps

AEP Ohio's CES project is expected to be deployed and monitored through December 2013. Metrics are being developed to ascertain the benefits the CES units provide to the distribution system and will be reported to the DOE as part of the project.

Emeka Okafor (cokafor@aep.com) is an engineer in American Electric Power's research programs department in Columbus, Ohio, U.S., and is responsible for researching new energy storage technologies and assessing them for deployment at AEP. He is currently managing the deployment of the utility's 2-MW, 2-MWh lithium-ion based community energy storage systems in Ohio. He holds a BE degree from Oral Roberts University and a MSEE degree from Oklahoma State University.

Editor's note: This material is based upon work supported by the Department of Energy under award number DE-OE0000193. This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer or otherwise does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

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